Because the most important parts of the expertise are coming from their internal "world model" and are inseparable from it.

An average unaware person believes that anything can be put in words and once the words are said, they mean to reader what the sayer meant, and the only difficulty could come from not knowing the words or mistaking ambiguities. The request to take a dev and "communicate" their expertise to another is based on this belief. And because this belief is wrong, the attempt to communicate expertise never fully succeeds.

Factual knowledge can be transferred via words well, that's why there is always at least partial success at communicating expertise. But solidified interconnected world model of what all your knowledge adds up to, cannot. AI can blow you out of the water at knowing more facts, but it doesn't yet utilize it in a way that allows surprisingly often having surprisingly correct insights into what more knowledge probably is. That mysterious ability to be right more often is coming out of "world model", that is what "expertise" is. That part cannot be communicated, one can only help others acquire the same expertise.

Communicating expertise is a hint where to go and what to learn, the reader still needs to put effort to internalize it and they need to have the right project that provides the opportunity to learn what needs to be learnt. It is not an act of transfer.